Overview on Methodology for Clean Air Study

By Michael Greenstone

Sept. 23, 2015

In our article on the connection between cleaner air and longer lives, we calculated the average increase in life expectancy associated with declines in air pollution for United States metropolitan and micropolitan areas (CBSAs) between 1970 and 2014.

We impute the total decrease in total suspended particulates (TSPs) for each geographic area where TSPs is not observed by calculating county-level ratios between TSPS, and particulate matter up to 10 µm (PM10) and 2.5µm (PM2.5). We convert TSPs decline to life-years saved using an estimate of the elasticity of life expectancy to particulate air pollution from Chen et al. (2013).

This exercise requires two crucial assumptions:

1) The E.P.A. monitors different pollutants over time — before 1990, the E.P.A mostly monitored TSPs; during the 1990s the E.P.A. mostly monitored PM10; and after 2000 the E.P.A. mostly monitored PM2.5. To create a comparable pre- and post- period, we impute TSPs when PM10 or PM2.5 data is available. Our preferred imputation scheme is as follows: We use TSP readings when available, imputed TSP from county-specific PM10 to TSP ratios when PM10 is nonmissing and PM2.5 is missing, imputed TSP from county-specific PM2.5 to TSP ratios when PM2.5 is nonmissing and PM10 is missing, and whichever pollutant has more overlap years for a given county when both PM10 and PM2.5 are available. The results are generally robust to alternative imputation schemes. Our imputation scheme does require stability of PM10/PM2.5 to TSPs ratios over time.

2) Air pollution is constant within CBSAs[1]. We assign TSPs levels to CBSAs as long as there is a single county with monitoring data in our pre- and post- periods. This requires that sampled counties are representative of out-of-sample counties. Our sample of monitored counties covers 125 million people. Extrapolating to CBSAs results in a population of 203 million people.

[1] The term “CBSA” refers collectively to both metropolitan statistical areas and micropolitan areas. Micropolitan areas are based on Census Bureau-defined urban clusters of at least 10,000 and fewer than 50,000 people — metropolitan areas have greater than 50,000 people. CBSAs are composed of county or county-equivalent entities.